1. Field of the Invention
The present invention relates generally to an optical transmission terminal device used in an optical communication field, and more particularly to a package structure of an optical module for performing conversion from an optical signal to an electrical signal or conversion from an electrical signal to an optical signal.
2. Description of the Related Art
In a recent information communication field, high-speed large-capacity computing and high-speed data transmission are required in response to advance of information. To meet this requirement, optical transmission is indispensable and it is now being prepared for an expansion and proliferation of optical communication networks.
As a device used at various sites in an optical transmission system, there exists an optical transmission terminal device including an optical circuit and an electrical circuit in combination to perform conversion from an optical signal to an electrical signal or conversion from an electrical signal to an optical signal. At present, the scale of production of the optical transmission terminal device per manufacture is about a hundred thousand a year. However, it is said that the production scale required in the future will become a million or more a year in response to a proliferation of optical communication networks and that the manufacturing cost will have to be reduced to 1/10 or less of the present level. To this end, it is strongly desired to develop a form of the optical transmission terminal device which can realize mass production and low cost by minimizing the number of parts and simplifying the fabrication process and can also ensure high reliability and long life.
A component mounted on a printed wiring board incorporated in communication equipment is generally classified into a surface mount type and a through-hole mount type. A typical example of the surface mount type component is an LSI, which has a form called a flat package. This component is soldered by a reflow soldering process. This process is carried out by printing a solder paste on a printed wiring board, attaching the surface mount type component to the printed solder paste, and soldering the component to the printed wiring board in a conveyor furnace giving a solder surface temperature of 220.degree. C. or higher.
On the other hand, a typical example of the through-hole mount type component is a large-capacity capacitor or an LSI having a large number of terminals (200 or more terminals). The LSI having a large number of terminals has a terminal form called a PGA (Pin Grid Array). Such a through-hole mount type component is soldered by a flow soldering process. This process is carried out by inserting the terminals of the through-hole mount type component into through holes of a printed wiring board, dipping the printed wiring board into a solder bath at about 260.degree. C. from the side opposite to the component mounted side, and soldering the component to the printed wiring board in the solder bath.
In the case of mounting an optical module on a printed wiring board by soldering similar to that used in mounting the surface mount type component or the through-hole mount type component, a so-called pigtail type optical module with an optical fiber cord is unsuitable. Usually, the optical fiber cord has a nylon coating, which has a low heat-resisting temperature of about 80.degree. C. Accordingly, the optical fiber cord is melted in the soldering process. Further, the optical fiber cord itself is inconvenient for storage and handling at a manufacturing site, causing a remarkable reduction in mounting efficiency to a printed wiring board. Accordingly, to allow the use of soldering for an optical module and reduce a manufacturing cost, the application of a so-called receptacle type optical module is essential.
A conventional receptacle type optical module allowing the use of soldering is described in IEICE, General Meeting, papers C-184, 1996 (Reference 1). The receptacle type optical module described in Reference 1 has a structure such that a photoelectric converter and an optical fiber with a ferrule are held on a silicon substrate, and this assembly is packaged by a ceramic.
In this optical module structure, a cover is mounted on the ceramic package and fixed by a thermoplastic resin adhesive to achieve hermetic sealing of an optically coupled portion, so as to prevent corrosion of the photoelectric converter due to moisture, oxygen, etc. and condensation at the optically coupled portion. Further, a block as an optical fiber holding member is mounted on the ceramic package to allow connection and disconnection of the ceramic package to a second optical fiber. Flat leads extending from the ceramic package are soldered to a printed wiring board by reflow soldering, thereby achieving mounting of the optical module on the printed wiring board.
Another conventional receptacle type optical module is described in IEICE, General Meeting, papers C-207, 1996 (Reference 2). The receptacle type optical module described in Reference 2 has a structure such that a photoelectric converter and an optical fiber with a ferrule are held on a silicon substrate and covered with a silicon cap for the purpose of hermetic sealing an optically coupled portion, and this assembly is fully molded with an epoxy resin. A commercially available MU type connector housing is mounted on an optical fiber connecting portion of the optical module to allow connection and disconnection of the optical module to a second optical fiber. Further, leads extending from the molded package are soldered to a printed wiring board by flow soldering, thereby achieving mounting of the optical module to the printed wiring board.
The most significant challenge in the optical transmission terminal device is to achieve low cost. Much of the cost is related with an optical module having a photoelectric conversion function. It is therefore essential both to ensure high performance, high reliability, and long life of the optical module and to simplify and make efficient the fabrication process of the optical module and the mounting method for the optical module to a printed wiring board. However, the above-mentioned prior art has the following problems.
In the optical module described in Reference 1, the package and the cover cooperate with each other to form a structure for hermetically sealing the optically coupled portion. However, a side wall of the package is formed with a slit for taking the optical fiber out of the package, so that a gap in the slit must be closed to realize the hermetic sealing. Accordingly, a step of filling the gap with an adhesive is required, and this step is unavoidably manually performed, causing a reduction in fabrication efficiency.
Further, the optical fiber with the ferrule is constructed by using a bare fiber and a ferrule as separate components, partially inserting the bare fiber into the ferrule, and fixing them together by an adhesive. Accordingly, a stress is readily applied to a root portion of the optical fiber (i.e., a boundary portion between the bare fiber and the ferrule), and the optical fiber possibly cannot ensure a load in connection or disconnection of an optical fiber connector to the optical module. Further, this optical module requires the package for hermetically sealing the optically coupled portion, and an expensive ceramic package is used as this package. Accordingly, there is a limit from the viewpoints of reduction in parts count and reduction in material cost.
On the other hand, the fabrication of the optical module described in Reference 2 requires the step of molding the optically coupled portion hermetically sealed by the silicon substrate and the silicon cap with an epoxy resin having a low coefficient of thermal expansion. In this molding step, a pressure as high as 80 kgf/cm.sup.2 is applied to the optically coupled portion in injecting a molten resin into a mold. In the case that the optical module is a semiconductor laser module, a tolerable misalignment between the semiconductor laser and the optical fiber is usually very exact such as .+-.1 .mu.m or less, and it is accordingly very difficult to maintain the position accuracy between the photoelectric converter and the optical fiber in the resin molding step involving application of the above-mentioned injection pressure. As a result, the optical modules manufactured exhibit large variations in optical coupling loss, which lead to a reduction in yield. Further, since the semiconductor laser is surrounded by the resin material having low heat conductivity, a deterioration in characteristics of the semiconductor laser module requiring heat dissipation is unavoidable.
Each of the optical modules described in References 1 and 2 has a form such that an optical fiber connector is plugged into the optical module toward its side surface in one direction. Further, the connection or disconnection of the optical fiber connector is carried out after soldering the optical module to the printed wiring board. Accordingly, when connecting or disconnecting the optical fiber connector, a stress is concentrated at a soldered portion between the optical module and the printed wiring board via the leads. As a result, there is a possibility of electrical contact failure caused by solder separation due to the stress or lead break due to metal fatigue, for example.